1. Field of the Invention
The invention relates to an organometallic complex, and more particularly to a green phosphorescent iridium complex applied in organic light-emitting diodes and fabrication method thereof.
2. Description of the Related Art
The first high efficiency organic electroluminescent device was developed by Kodak Corporation in 1987. Since then, organic (polymer) electroluminescent devices have been widely applied. Organic electroluminescent devices are applied in flat panel displays due to their high illumination, light weight, thin profile, self-illumination, low power consumption, wide viewing angle, simple fabrication methods and rapid response time. Additionally, when applying organic electroluminescent devices in flat panel displays, no backlight modules in the flat panel displays are required.
The theory of electroluminescence is described as follows. When an external electric field is applied to an organic semiconductor thin film device, electrons and holes are injected from a cathode and an anode, respectively, transported and then recombined to form excitons in an emitting layer. Energy is further transported from the excitons to luminescent molecules under a continuous electrical field. Finally, the luminescent molecules emit light converted from the energy. A common organic electroluminescent device structure comprises an ITO anode, a hole transport layer evaporated on the ITO anode, an emitting layer evaporated on the hole transport layer, a hole blocking layer evaporated on the emitting layer, an electron transport layer evaporated on the hole blocking layer, and a cathode evaporated on the electron transport layer. A multiple-layered organic electroluminescent device may further comprise a hole injection layer formed between the anode and the hole transport layer or an electron injection layer formed between the cathode and the electron transport layer evaporated from a proper organic material to improve carrier injection efficiency, thereby reducing driving voltage and increasing carrier recombination.
When luminescent molecules absorb energy to achieve an excited state, fluorescence or phosphorescence illumination is subsequently emitted. Fluorescence illumination is emitted via radiation transition from a singlet excited state to a ground state. Phosphorescence illumination is emitted via a radiation transition from a triplet excited state to the ground state. In a fluorescence electroluminescent device, 75% of the excitons form by recombination of electrons and holes to achieve a triplet excited state, the remaining 25% of the excitons do not form due to spin forbidden effects. Additionally, there is no illumination when the excitons transit from the triplet excited state to a ground state. Thus, a fluorescence electroluminescent device has only 25% internal quantum efficiency, which substantially limits its external quantum efficiency to lower than 5%. Accordingly, phosphorescent materials have been developed to fully utilize the characteristics of the triplet excited state transition to improve the internal quantum efficiency of electroluminescent devices from 25% to 100%. For example, red phosphorescent material is prepared by doping phosphorescent dyes in a host. Energy is transported from the host to the phosphorescent dyes through excitons for illumination. The most preferably dopant used is a phosphorescent dye containing heavy atoms. A heavy atom effect improves electron spin-spin coupling. With improved electron spin-spin coupling, the singlet state and triplet state are more effectively mixed and the probability of crossing between the singlet state and the triplet state is increased. Thus, the life span of the triplet excited state is reduced and the luminescent efficiency of the phosphorescent material is improved by four times that of the fluorescencent material.
Fac-Ir(ppy)3 is a green phosphorescent material doped in a CBP host. A device utilizing fac-Ir(ppy)3 can achieve a maximum external quantum efficiency of 8.0% (28 cd/A) and a luminescent efficiency of 31 lm/W and a maximum emitting wavelength of 510 nm and CIE of (0.27, 0.63). Specifically, the life span of the triplet excited state of fac-Ir(ppy)3 is merely about 2 μs at room temperature, effectively reducing the saturation of devices under high current density. The structure of the fac-Ir(Ppy)3 device has been further optimized by Watanabe et al. When a doping concentration of fac-Ir(ppy)3 is 8.7% for a fac-Ir(ppy)3 device, under an illumination of 100 cd/m2, the fac-Ir(ppy)3 device can achieve an external quantum efficiency of 14.9% and luminescent efficiency of 43.31 lm/W, which is about two times that of the original fac-Ir(ppy)3 device. Meanwhile, (ppy)2Ir(acac) is another green phosphorescent material commonly used. An ITO/HMTPD/(ppy)2Ir(acac):TAZ/Alq/Mg:Ag device has a maximum emitting wavelength of 520 nm and CIE of (0.31, 0.64) and maximum external quantum efficiency of 19% due to improved control of balance between the electrons and holes and luminescent efficiency of 60 lm/W. Certainly, nearly 100% phosphorescent efficiency of (ppy)2Ir(acac) is also an indispensable characteristic to prepare high-efficiency devices. A series of compounds which comprises iridium and a series of benzoimidazole-containing derivatives serving as ligands have optimal thermal stability. A yellow-green pbi2Ir(acac) device has a maximum emitting wavelength of 530 nm, CIE of (0.36, 0.60), maximum quantum efficiency of 16.7% and maximum luminescent efficiency of 20 lm/W.
Other phosphorescent materials which comprise an iridium center and various ligands have been synthesized. The luminescent property effect, by the alternating of various ligands, such as emitting wavelength and luminescent efficiency has been disclosed, for example in EPO 1,434,286. In US 2002/024293, a blue phosphorescent iridium complex material with an emitting wavelength exceeding 500 nm is disclosed. The device has an external quantum efficiency exceeding 5%. In US 2002/034656 and 2003/017361, a platinum complex with various emitting wavelengths of 425 nm, 475 nm, 500 nm, 575 nm and 615 nm, ranging from blue light to orange-red light, is disclosed.